1. Field of the Invention
The present invention relates generally to the interconnect structures contained within an integrated circuit device. More particularly, the present invention relates to a multilayer interconnect structure that uses layers of titanium, titanium nitride and aluminum-copper alloy to improve electromigration reliability.
2. Description of the Prior Art
Integrated circuits generally comprise a semiconductor substrate upon which are formed various electronic components such as transistors, diodes and the like. Interconnect layers are formed on the semiconductor substrate to electrically interconnect the various electronic components to each other and to external components. Traditionally, the interconnect layers used on the semiconductor substrate have been made from polysilicon films, high temperature metal films, metal silicide films, aluminum films and aluminum alloy films. Each of these interconnect layers have an inherent resistance. The performance characteristics of highly integrated, high speed integrated circuits require that the resistance within the interconnect layers be held to a minimum. As a result, high speed integrated circuits typically use interconnect structures made of aluminum films or aluminum alloy films that have a relatively small resistivity as compared to the other interconnect structure choices.
In the prior art, it has been shown that the use of an aluminum film or an aluminum alloy film directly on a silicon-based semiconductor substrate is highly problematic. When the aluminum comes into direct contact with the silicon-based semiconductor substrate, an adverse electromigration reaction occurs causing an alloy spike in the semiconductor substrate. The alloy spike may extend through the region of the impurity diffusion layer and extend downwardly into the semiconductor substrate, thereby resulting in a junction leak at the impurity diffusion layer. In order to prevent the degradation of the impurity diffusion layer, a barrier film is commonly deposited between the silicon-based semiconductor substrate and the aluminum containing interconnect structure.
The prior art is replete with references that show the use of a barrier film between an aluminum alloy interconnect structure an underlying semiconductor substrate. Such prior art references are exemplified by U.S. Pat. No. 5,278,099 to Maeda, entitled METHOD FOR MANUFACTURING A SEMICONDUCTOR DEVICE HAVING WIRING ELECTRODES, and U.S. Pat. No. 5,313,101 to Harada et al., entitled INTERCONNECT STRUCTURE OF SEMICONDUCTOR INTEGRATED CIRCUIT DEVICE. Both of these patents disclose the use of titanium nitride (TiN) as a barrier film.
Although barrier films have been used, electromigration still occurs between the aluminum alloy interconnect structure and the underlying silicon-based semiconductor substrate. In order to limit the amount of electromigration induced failure in an integrated circuit, the current density is limited within the interconnect structures. The imposed current limitations on the interconnect structures limit the design flexibility and circuit density supported by integrated circuit technologies. The problems associated with electromigration become more severe with the small linewidths and line thicknesses used in advanced integration technologies. As a result, the limitations of prior art interconnect structures impact the reliability of the overall circuit, as well as chip density and performance.
It is, therefore, an objective of the present invention to provide an interconnect structure for an integrated circuit that prevents alloy spiking of the below lying substrate and has greatly improved electromigration reliability, thereby enabling integrated circuits that are more reliable, have higher performance rates and have increased chip density.